1. Field of the Invention
This invention relates to a scanning optical apparatus for use in an image forming apparatus such as a laser beam printer or a laser facsimile apparatus, and particularly to a light source unit integrally holding a light source and light detecting means.
2. Description of Related Art
FIG. 1 of the accompanying drawings shows a scanning optical apparatus (see, for example, Japanese Patent Application Laid-Open No. 2000-131634) used in a conventional image forming apparatus. A laser beam emitted from a semiconductor laser 101 is converted into a parallel beam or a convergent beam by a collimator lens 102, and is imaged in a linear shape on a rotary polygon mirror 104 by a cylindrical lens 103. This laser beam is then deflected by the rotary polygon mirror 104, and is imaged and scanned on a photosensitive drum 106 by an Fθ lens 105. Also, part of the deflected laser beam is incident on a light receiving element 108 through a synchronization detecting lens 107 by the utilization of a portion outside an image area, and writing start position adjustment is effected on the basis of the signal of the light receiving element 108. The light receiving element 108 is carried on the same substrate as a circuit substrate 100 on which the semiconductor laser 101 is carried, whereby it is unnecessary to provide a discrete circuit substrate exclusively for the light receiving element 108. The provision of a discrete circuit substrate would lead to an increase in the number of parts and a plurality of substrates, which in turn would lead to the complication of an electric circuit substrate, and this would result in a factor of increased costs such as an increase in the work of running wiring and therefore, Japanese Patent Application Laid-Open No. 2000-131634 has been proposed as an improving means.
In recent years, the scanning optical apparatus of the above-described construction has come to be designed such that toward a still higher speed, the rotary polygon mirror 104 is rotated at a high speed. Accordingly, the time from after the first laser beam has been detected by the light receiving element 108 until the next laser beam is detected by the light receiving element 108 becomes shorter as the rotary polygon mirror 104 is rotated at a higher speed. In this case, the light receiving element 108 need be quick in response and for this purpose, the light receiving surface of the light receiving element 108 is required to be as small as possible.
Consequently, if the light receiving surface of the light receiving element 108 is large, the relative positional relationship between the light receiving element 108 and an optical element (such as, for example, the rotary polygon mirror 104 or the synchronization detecting lens 107) for directing the laser beam to the light receiving element 108 need not be so strict, but yet if the light receiving surface of the light receiving element 108 is small, there has been the possibility of the laser beam being incapable of being detected by the light receiving element 108 unless the relative position of the light receiving element 108 and the optical element is determined accurately.
Also, when as described in Japanese Patent Application Laid-Open No. 09-230259 or Japanese Patent Application Laid-Open No. 2002-189180, a multilaser beam provided with a plurality of light emitting points is used as a semiconductor laser, a laser holder must be rotated and adjusted for the adjustment between the pitches in the sub-scanning direction, and at that time, a BD sensor (light receiving element) is integral with the laser holder and therefore, when the laser holder is rotated about an optical axis, the BD sensor is also rotatively moved in the same direction, and it has been difficult to introduce scanning light from a light deflector into the BD sensor.